In the current automobile industry, the tendencies are toward higher engine power, and smaller size and lighter weight to ensure the living space in the car, and the production of front-engine front-drive (FF) vehicles is therefore increasing. In the FF vehicles, it is required that the engine power be transmitted to the front wheels smoothly. The constant velocity joint becomes an essential part to transmit the rotational motion to the two shafts at a constant velocity even if the two shafts intersect at any angle.
The constant velocity joint continues a complicated rolling—sliding motion under a high contact pressure while rotating. A stress is repeatedly applied to the lubrication portion which is in contact with the metal member, so that further improvement of the antiwear properties and the anti-flaking properties under the severe lubrication conditions has been demanded.
There is known a grease composition (for example, as in JP 2002-20776 A) capable of reducing wear of the constant velocity joint, in particular, the ball-type constant velocity joint, to prevent the flaking problem, which comprises an overbased sodium sulfonate, a sulfur-containing extreme-pressure agent such as molybdenum dialkyl dithiocarbamate or the like, and a phosphorus-containing antiwear agent.
There is also known a grease composition for constant velocity joints (for example, as in JP 2006-335876 A) capable of effectively reducing wear and friction of the joint to prevent the vibration from occurring, which comprises melamine cyanurate and molybdenum dithiocarbamate.
A grease composition comprising an alicyclic or aromatic diurea, melamine cyanurate, molybdenum disulfide and a sulfur-containing extreme-pressure agent free from phosphorus is also known (for example, as in JP Hei 09-255983 A), which can optimize wear of the constant velocity joint to prevent the flaking problem, and at the same time, exhibit excellent heat-resistance.
A grease composition as disclosed in JP 2008-297402 A comprises diurea, zinc dialkyldithiophosphate, molybdenum-dialkyldithiocarbamate, zinc dialkyldithiocarbamate, and a sulfur-nitrogen based extreme-pressure agent for improving the flaking problem of the constant velocity joint.
A grease composition for the ball-type constant velocity joint as disclosed in JP 2011-236354 A comprises a diurea thickener, a base oil, zinc dialkyldithiophosphate, molybdenum dialkyldithiocarbamate, melamine cyanurate, calcium carbonate, zinc dialkyldithiocarbamate, and a sulfur-nitrogen based extreme-pressure agent for improving the antiwear properties under a high contact pressure to increase the anti-flaking properties.
A grease composition as disclosed in JP 2003-82374 A comprises a base oil, a diurea compound and benzotriazole and/or a derivative thereof, which grease composition is not designed for the constant velocity joint, but is capable of extend the flaking life of a lubricated steel portion subject to the rolling motion and the rolling—sliding motion.
However, the antiwear properties and the anti-flaking properties that can be obtained by the above-mentioned grease compositions are not considered to be sufficient.